Herbert Blum reports on leakage currents in fault-current protected environments.
To provide for protection of personnel, residual current operated circuit breakers are seeing increased use in electrical installations. These often trip unnecessarily due to leakage currents caused by electrical systems. The result is machine downtime that, however, can be prevented – with knowledge about leakage currents and targeted countermeasures to ensure efficient yet safe operation. Because power-line filters are significant causes of ground currents, they deserve special attention.
If a person directly touches a live conductor, the fault current flows to ground. An upstream RCD (residual current protective device) detects this fault current and immediately disconnects the circuit.
But a RCD cannot distinguish between normal residual currents and dangerous fault currents. Leakage currents can thus trigger a RCD.[Page Break]
In EMC filters, capacitors from all conductors are wired to ground. Current is continually flowing through each of these Y-capacitors. In an ideal 3-phase power network with sinusoidal voltages, the sum of all these currents is zero. In practice, however, there is a continuous leakage current to ground due to strong distortion in the grid voltage. Most filter manufacturers specify the maximum expected leakage current so that it is easier to select a suitable filter. But these are theoretical values, which can deviate due to unsymmetrical loading or a higher frequency (>50Hz). Thus, it is advisable to measure current to ground with filters installed and in operation.
Many frequency inverters are delivered with integrated filters (footprint filters). These are generally simple filters with small chokes and large capacitors between the phase conductors and ground that cause large leakage currents. The filtering effect of the large Y-capacitors can generally be replaced only with larger inductances. For example, a one-stage filter with large Y-capacitors must be replaced with a two-stage filter with two chokes.[Page Break]
Often there is also an EMC statement of conformity for the enclosed filters. This, however, is valid only for an ideal installation and short motor cables. Longer motor cables require a new EMC measurement. They also generate a larger capacitance to ground, which can result in larger leakage currents. These additional asymmetrical currents can lead to the magnetic saturation of the filter chokes. As a result, the filter loses a large part of its effectiveness and the system exceeds the permitted EMC limits.
A remedy can be provided with shorter cables or an output filter. This sine-wave filter should be inserted directly at the inverter’s output. It effectively attenuates leakage currents above 1 kHz by reducing the slew rate of the motor voltage.
If multiple inverters are used in a system, it can be worthwhile to use a common filter at the grid input instead of a filter for each inverter. Many manufacturers also offer special low leakage-current filters for their inverters or summation filters for use at the grid input.
An especially simple option is to use a 4-conductor filter with a neutral conductor instead of a 3-conductor filter. Most filters with a neutral conductor have smaller leakage currents because many capacitors are connected between the phase conductors and the neutral conductor. In this way, most of the leakage current returns through the neutral conductor. Because the neutral conductor is measured by the RCD in the same way as the phase conductors, the device does not trip because the sum of the currents is equal.
If a filter does not have sufficient attenuation, it can be combined with an additional power-line choke. This reduces the current’s ripple factor along with harmonics and thus provides for smaller leakage currents.
Herbert Blum is Product Manager EMC, SCHURTER AG, Lucerne, Switzerland. www.schurter.com
Fig. 1: RCDs disconnect the circuit and thus prevent accidents (source: SCHURTER AG)
Fig. 2.SCHURTER sine-wave filters series FMAC SINE (source: SCHURTER AG)